Though the average salt concentration of the ocean is around 35 parts per thousand, there is a lot of variability in salinity from basin to basin and across time and space. Shown here is a model of surface salinity generated by the NOAA Environmental Modeling Center. Visible are the differences between salinity in the Western Atlantic and the Eastern Pacific. This variation in surface salinity arises from differing freshwater inputs and evaporation within the respective basins. Fresh water evaporates from the warm tropical waters of the Atlantic and is transported through the atmosphere as water vapor, eventually precipitating in the Pacific. The large moisture plumes in the Pacific are also carried northward via atmospheric rivers. Because the California Current (along the U.S. west coast) is bringing cold water down from the north, it is not subject to as much evaporation as the warm waters carried northward by the Gulf Stream along the east coast. Coastal river input also drives down surface salinity – which can be seen in off Washington, parts of the Mid-Atlantic, and in the Gulf of St. Lawrence.